Support OMM file format

As documented at Celestrak’s website, an XML/JSON based format called OMM (spec: https://public.ccsds.org/Pubs/502x0b2c1.pdf) is being pushed as a more modern replacement for the TLE format. Since the parameters are still reported in terms of sgp4 elements, and given the dominance of CelesTrak as a source of orbit information, it seems reasonable imo to add support to this project for the OMM format.

I’d be happy to open a PR for this with some guidance on how you’d like this structured. Would it be more reasonable to:

1. Add this functionality into io.py?
2. Create a new submodule, omm.py or something similar?
3. Something else?

This is my first time trying to contribute to an open-source project, so apologies if there’s something I’m missing.

[brandon-rhodes]

I have just released a new version of the library that, thanks to some code contributed by @astrojuanlu, can do OMM export to a Python dictionary, which the user is then free to output as JSON or CSV or more exotic formats as they please. While users might run into snags that will require a bit more documentation or code, I think that we can now close this issue, since the basic round-trip to and from OMM is now possible.

Feel free to make further comments here in this issue, though, if you have questions about the new support that don’t seem to warrant an entire new issue!

And thanks again, everyone, for pointing my attention at the new format, which hadn’t been on my radar yet.

[brandon-rhodes]

@benjaminglass1 — I don’t see any useful Python code at https://spacedatastandards.org/ and I don’t see any update for the traditional SGP4 C++ library to support the new formats, so it does look appropriate for this library to add code itself for reading the format.

There does seem to have been some code for this feature, though, offered last week over on the Skyfield project that uses this library:

https://github.com/skyfielders/python-skyfield/pull/398

I’ll see about getting that author to instead make a pull request here — in which case there might not be much new code needing to be written, @benjaminglass1; we’ll see!